Prepare the filling: Add the potatoes and garlic to a large pot, cover with cold water, and add 2 teaspoons of salt. Cover and bring to a boil. Cook for 10 minutes, until the potatoes are easily cut with a fork. Drain and put through a potato ricer or pass through a fine sieve. Transfer to a large bowl. 

While the potatoes cook, melt the butter over medium heat in an 8-inch nonstick skillet. Add the onions and 2 teaspoons of salt, and cook for 10 minutes until golden. Combine with the potatoes, add pepper, and adjust seasoning as needed. Set aside. Wipe out the skillet and set aside.

Make the crepes: In a large bowl, whisk the flour, milk, seltzer, eggs, and 2 tablespoons of melted butter until no lumps remain.

Spray the skillet with non-stick cooking spray or melt 1 tablespoon butter in the same 8-inch nonstick skillet over medium-low heat. Pour a ¼ cup of batter into the skillet and tilt to spread the batter in a very thin layer. Make sure to spread the batter evenly around the pan. Cook 2 minutes per side, until set. Transfer to a plate lined with parchment, and repeat with remaining batter. Set the skillet aside.

Assemble the blintzes: add ¼ cup of the filling in a line near the bottom edge of the crepe. Fold the lower edge up and over the filling. Then, fold each side in about 1-inch, and roll up, ending with the seam side facing down. Repeat with remaining filling and crepes.

Add ¼ cup of oil to the skillet and heat over medium. Working in batches, gently fry the blintzes for 1-2 minutes on each side, until they are lightly browned. Transfer to a cooling rack or paper towel-lined plate to drain excess oil. Serve warm, with sour cream and chives.